<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a {
            border: 1px solid #333;
            width: 100px;
            height: 100px;
            position: relative;
        }

        .s {
            position: absolute;
            left: 50%;
            top: 50%;
            margin-top: -25px;
            margin-left: -25px;
            width: 50px;
            height: 50px;
            background-color: pink;
        }
    </style>
</head>

<body>
    <div class="a">
        <div class="s">

        </div>
    </div>
</body>

</html>